Audion circuit



Feb. 9, 1932. 5, 4 BA LLA 11 N E r 1,844,456

AUDION CIRCUI T Filed Nov. 3, .926 3 Sheets-Sheet l S BALLANTINE AUDIGN GIRUUI'T Feb. 9, 1932.

Filed Nov. '5. 1926 3 Sheets-sneet 2 Feb. 9, 1932. 5 BALLANTmE 1,844,456

AUDION CIRCUIT Filed Nov. 3, 1926 3 Sheets-Sheet 3 Patented Feb. 9, 1932 UNITED STATES PATENT OFFICE STUART BALLANTINE, 01 WHITE HAVEN, PENNSYLVANIA, ASSIGNOR T BOONTON' RESEARCH CORPORATION, OF BOONTON, NEW JERSEY, A CORPORATION OF NEW JERSEY AUDION Application filed November This invention relates to an electrical circuit for use in association with audion tubes, and especially to such a circuit providlng means for reducing or preventing reaction between the output circuit of an audion stage and the input circuit thereof. It is applicable to audion stages in general, including audion amplifier stages suitable for use in radio receiving sets, or in radio or intermediate frequencyamplifiers whether or not em-- bodied in a radio receiving set.

In my copending application, Serial No. 629,702, filed April 3, 1923, I have heretotlore disclosed and claimed an electrical circuit for use in association with audion tubes application relates particularly to a class of alternating current bridge circuits having a balancing arm of the bridge mesh between grid and plate of the audion tube and constituting a substantially pure capacity. My jiiresent invention relates to a class of bridge circuits having a balancing arm between grid and plate of the audion tube and comprising an in ductance.

T have :tound that by arranging the impcdance mesh of an audion stage in the form 9 ol? an alternating current heatstones bridge of which the input, or isolated, circuit of the audion stage forms one conjugate arm, the output, or plate-filament, circuit of the audion s age forms the other conjugate arm, and one balancing arm comprises inductance between grid and plate of the audion tube, I obtain an audion stage of which the output circuit may be electrically isolated from the input circuit in such a way that there is substantially no reaction between them. By appropriately balancing the bridge mesh such reaction may be diminished to the desired degree. The audion stage thus approaches a unidirectional repeater. Any desired number of such stages may be cascaded together CIRCUIT a, 1926. Serial No. 146,038.

without reacting one upon the other, and in the case of amplifier stages the amplification obtained from the cascaded stages increases in substantially geometrical progression.

Fig. 1 represents diagrammatically a general of a VVheatstones bridge mesh associated with an audion tube. The output circuit of the audion stage is between the plate and filament oi the audion tube and forms one conjugate arm of the bridge. The input circuit of the audion stage is not between grid and filament 01"" the tube but is between the grid of the tube and an intermediate point, 2, in the output or plate-filament circuit. The input circuit of the stage therefore forms the other conjugate arm of the bridge, and when the bridge mesh is balanced the input circuit of the audion stage is electrically isolated from the output circuit thereof. I may therefore designate the input circuit of the stage as the isolated circuit, since it is by virtue of the electrical isolation of this input circuit from the output circuit that reaction from one to the other is prevented.

In Figs. 2a and I have diagrammatically shown two circuits each embodying my invention. That is, in each case the impedance mesh of the audion stage is arranged as a Whcatstones bridge having four balancing arms and two electrically isolated conjugate arms, and the balancing arm between grid and plate of the audion tube comprises the inductance Lin. In physical embodiments of the circuit this inductance is efiectively shunted by the distributed capacity of the coil and by the inherent grid-plate capacity of the audion tube, but I prefer to use a coil of such size that the reactance of this balancing arm is inductive for frequencies at which the circuit is intended to operate.-

The principles of electrically balancing an alternating current bridge mesh are of course familiar to those skilled in this art. In gen eral they require the use oi another inductive balancing arm adjacent Lm, and further require that the remaining two balancing arms comprise impedances of a character (inductive, capacitive, or resistive) similar to one another, although they may be of the same or difi erent character from that of the arm Lm.

'quency in the same manner. it is advantageous to couple elther the input An exception, however, occurs in the circuit of Figs. 11a and 115, which is discussed below.

The arm Lm having already been determined, there are two positions in which the adjacent inductive arm may be placed. In

Fig. 2a it is shown at L1 and in Fig. 27) at Le.

In Fig. 2a there remain two balancing impedances Z0 and Z2 as yet undetermined. These must be similar in character in order to give a balance, but they may be either capacitive, resistive, or inductive. These three alternatives are shown in Figs. 3a, 3b, and 30, respectively. As already'pointed out above, the inductance Lm is shunted by the internal grid-plate tube capacity Om, in addition to any distributed coil capacity which may be present. In order to secure abalance inde pendent of frequency it may therefore be desirable toshunt the adjacent inductive arm by a capacity of proper order. This is indicated at Cm and C in Fig. 3a, but to simplify the diagrams it has been omitted from the other figures. V

The alternative shown in Fig. 3a has a particular advantage, that regeneration may take place in the audion tube while the input and output circuits of the audion stage are maintained isolated from one another by the bridge mesh. This is applicable either to an oscillator or to an amplifier, and in the latter case a high amplification is obtained from each stage, while, any number of regenerative amplifier stages may be cascaded withoutrev acting upon each other. This has not heretofore been possible.

The arrangement of Fig. 26 leaves two undetermined impedances Z and Z which must fulfill the requirements already stated. They may therefore be both capacitive, both resistive, or both inductive. The firsttwo alternatives are shown in Figs. 4a and 4?) respectively. The third alternative gives the same circuit as Fig. 30 and is therefore not shown.

In order to obtain abridge balance independent of frequency it is necessary either that the ratio of the two impedances forming each pair of balancing arms be invariant of frequency, or that each pair of impedances be so selected that their ratiosvary with fre- In some cases circuit or the output circuit of the stage to two adjacent balancing arms of the bridge by means ofinutual inductance. The balancing arms then become composite because of the mutual inductance, but a balance may be maintained by virtue of the above principle. In Fig. 5a the input or isolated circuit is inductively coupled to the arms L0 and L2,

which thusbecome composite. In Fig.5b the output circuit is inductively coupled to the arms L1 and L2 in a similar manner. I have found that arrangements of this sort are Well adapted tense in commercial amplifiers.

Fig. 6a illustrates a particular application of my invention, making use of inductive coupling to obtain proper direct current paths. This is the circuit of Fig. 30. used as one stage of a tuned radio frequency amplifier. The isolated or input circuit comprises the tuned transformer T1. The. output circuit includes the tuned transformer. T2. This transformer has two primary windings P1 and P2 which may be closely associated so that the coupling approximates unity. A double winding is suitable and may act electrically as a single coil, the impedance between adjacent terminals being vanishingly small. In this'case' the two primary windings serve to supply the grid and plate electrodes with their proper operating direct current potentials by means of the batteries B77'and C72 V I Fig.6b illustrates another method of arranging the direct current battery paths. Here the direct current plate circuit is completed through the primary of transformer T2 and the directcnrrent plate potential is kept oil the grid by the insulating condenser C The proper grid bias is obtained by means of the resistance R0 and the battery C. This forms an arm of composite impedance which may bebalanced if necessary by connecting another resistance R2 across the adjacent capacitative arm C2 as shown.

Fig. 7 a shows the employment of inductive coupling in the balancing arms of the circuit of Fig. 3c. The windings P1 and P2 are closely coupled and constitute the primary of the output transformer of which the secondary is denoted by S. The inductive coupling serves to keepthe direct current plate potential of the battery B from the grid. The proper grid bias is obtained by the battery C. This type of amplifier stage is well adapted to cascade arrangements in which the filament junctions of the several bridge meshes are common.

Fig. 7b shows another arrangement of the A and B batteries, adapted to cascaded systems in which the intermediate points 2 of the successive bridge meshes are common. The filament circuit is complete through the portions P1 and P2 of the closely coupled primary windings. plin between the two primary windings Pl and 2 may be desirable but is not vital.

In cascading a plurality of these stages I may make any one of the four corners of the bridge mesh common to the several stages and ground this common point,it' I so desire. In commercial apparatusterminals F and 2 are perhaps most convenient for this purpose, and FigsQZa and'7b show suitable battery arrangements applied to the circuit of Fig.3c.

Figs.8a and 8?) show battery arrangements suitablefor'use in cascaded systems having Unity con bridn e meshes are made common. The li common plate terminals. These diagrams are also based upon the circuit of F 30. In Fig. 8a the filament circuit is completed around the direct current path fur l died by the four coils which constitute the bridge arms; in l 8?) it is complet d by means at the coil L 1 ch may or may not be coupled to the coi L1 and L2 of the balanc ng :1 ln Fig. 8a lay-pass condenser shown 11 parallel with the filament as a mean? inating the filament impedance brid e. This may not be neccssa; cases.

l ig. El sl'iows a section oi a cascaded tuned sternler-coupled aniplilier compounded is of the type shown in F 53. (3a. A desci-intion of:

A this has already been given so that the diagram is l l ero anator rho filament o't t be several laraents are connected in parallel and are suppl ed by the A batter". The lines to terv and C bimuig battery are a c l dense s (are byp coin t low inined :c their function be to prevent cohplin between stages due in common impcdances in the batteries and 'lheir eil'cct may be incre choke coils in the lines bet... condone; and the batteries.

nial by-pass condeu employed in tern: 7.1nher oi stages, 'li of the separate stages if d red.

t onnnonly controlled variable tuni= inents in t be several stages may also be us l'ustead of tuning: the coup ing tra ers by in is of the variable condensers shown. the condensers C0 and G2 may be used for this purpose by connect" them ther nnichanically so that their variaons of capacity talre place in such a way as to preserve their inipedances in. the proper ratio.

Fig. 10 shows single stage in which the coils hm and L1 are used as the secondary of a. coupling transformer. the couplings being indicated by braclte l ly properly coupling lam and lid a low impedance between G and E3 mav he obtained, while at the same time u'i'vi I the desirable high impedance between the co: nor of the mesh at P and G or For this coupling; the sign of the unitual inductance should be such that between the ter-- urinals G- and "he inductance oi? the two coils with an indiuttive balancing arm id and plate ot the audion tube. e two similar resistive arms are i this or p. positc one another instez d of adjacent, and the inductance Lot is balanced by means of a capacity G2 placed in the opposite arm. The condition for balance is well known to be Lm/C R R0. Fig. 116 illustrates a detailed circuit including batteries, and using inductive coupling to insulate the grid from the direct current potential of the plate.

My invention is obviously not confined to an audion tube oi the three-electrode type; It may extend to any vacuum tube stage having circuits which it is desired to isolate from one another. The application of my invention to four-electrode audion tubes of the type having two grid electrodes is shown in Figs. 12a and In the former, G2 is used as control grid and G is given a positive bias; in the latter G is used as control grid and G is given a positive bias.

in all oi the above circuits coupling the coils of the balancing); arms with one another and with coils in the conjugate arms is contemplated, and this may be carried out as already indicated or in a manner which will be apparent to one familiar with this art. Moreover my invention is not restricted to a completely balanced network out includes the combination cl an audion tube with a physical bridge structure as shown.

It is also ruidcrstood that the invention is not limited to the specific circuits which I have described since various changes or modifications may be made in the circuits without departing from the spirit of my invention.

1 claim:

1. An electrical amplifier stage comprising an audion tube and an impedance network associated therewith and arranged in the form of an alternating current lVheatstones bridge of which the input circuit of the audion stage forms one conjugate arm and the output circuit of the audion stage forms the other conjugate arin, said bridge having two adjacent balancingarms each comprising an inductance and ell'ectively located respectively between grid and plate of said tube and between. the plate of said tube and an intermediate point of an alternating current plate-filament path thereof, and two adjacent balancing arms each having acapacitive reactance and located in the two remaining balancing-ar1n positions in said bridge.

2. An electrical amplifier stage comprising an audion tube and an. impedance network associated therewith and arranged in the form of an alternating current Wheatstones bridge of which the input circuit of the audion stage forms one conjugate arm and the output circuit of the audion stage forms the other conjugate arm, said bridge having a balancing arm comprising an inductance located between the grid of said tube and a point in said bridge mesh which is electrically equivalent to, and associated with, the plate of said tube by virtue of a closely coupled twin winding. 7

3. An electrical amplifier stage comprising an audion tube and an impedance network associated therewith and arranged in the form of an alternating current Wheatstones bridge of which the input circuit of the audion stage forms one conJugate arm and the output circuit of the audion stage forms the other con ugate arm, sa1d bridge having two balancing arms each comprising an inductance, one of said balancing arms being eflectively located between grid and plate of said audion tube and the other of said balancing arms being effectively located adjacent thereto between the plate of said tube and an intermediate point in an alternating current path thereof, and capacitive impedance blocking the flow of direct current from said plate along either of said two balancing arms.

4:. An audion amplifier stage comprising an audion tube having a grid, a plate and a cathode, two inductances serially connected between plate and cathode, an inductance effectively connected between grid and plate, an inductance between grid and cathode, an output circuit for said stage effectively connected across plate and cathode, and a tuning condenser connected between grid and the junction of said serially connected inductances, said inductances constituting the arms of a lVheatstone bridge which may be balanced to isolate said condenser and said output circuit.

5. In a radio frequency amplifier stage, in combination, an audion tube having a grid, a plate and a cathode, inductive impedance connecting said cathode and grid, impedance connecting said cathode and plate, an

inductive impedance effectively connected between grid and plate, means preventing the direct current potential of said plate from reaching said grid, said impedances comprising the four balancing arms of a Vheatstone bridge network across which the input circuit and the output circuit of said stage are arranged as the conjugate arms thereof, whereby said bridge network may be balanced to suppress self-oscillation.

In testimony whereof, I afiiX my signature.

STUART BALLANTINE. 

